Broadband tower antenna system



Dec. 2, 1969 J. H. MULLANEY BROADBAND TOWER ANTENNA. SYSTEM 3Sheets-Sheet 1 Filed Aug. 29, 1966 HGia FIGBD INVENTOR JOHN H. MULLANEYBY 4 ,c9'd Mi,

ATTORNEYS Dec. 2, 1969 J. H. MULLANEY BROADBAND TOWER ANTENNA SYSTEM 3Sheets-Sheet 3 Filed m 29, 1966 INVENTOR JOHN H. MULLANEY FREQ-(KHZ) BY0757 a za.

ATTORNEYS Dec. 2, 1969 J. H. MULLANEY 3, 4

BROADBAND TOWER ANTENNA SYSTEM Filed Aug. 29, 1966 3 Sheets-Sheet 5FIGIO INVENTOR JOHN H. MULLANEY BY wfl a ATTORNEYS United States Patent3,482,249 BROADBAND TOWER ANTENNA SYSTEM John H. Mullaney, Potomac, Md.,assignor to Multronics,

Inc., Rockville, Md., a corporation of Maryland Filed Aug. 29, 1966,Ser. No. 575,613 Int. Cl. H01q 9/00 U.S. Cl. 343750 9 Claims ABSTRACT OFTHE DISCLOSURE A top-loaded, series-fed, low frequency antenna systemhaving a vertical radiator which is electrically short (20) incomparison to its operating wavelength, mounted on a base insulator andwherein the top loading comprises a plurality of staggered tuned dropwires terminated in the ground plane by respective adjustablecapacitors.

This invention relates to broadband radio antenna systems and moreparticularly to a transmitting antenna system of the vertical tower typewhich has a relatively short length in comparison to the desiredwavelength of operation.

When operating at low radio frequencies, electrically short verticalradiators are normally utilized due to the fact that quarter wavelengthtransmitting antennas for such frequencies become physically impracticaland extremely expensive. For example, at an operating frequency of 100kilocycles per second (kl-12.), a wavelength is 9,843 feet and thereforea quarter wavelength transmitting antenna would have a height ofapproximately 2500 feet above the ground. It is the practice, therefore,to employ antennas having electrical lengths in the order of 5 to 20,where 360 is defined as 1 wavelength. Acoordingnly, it is commonpractice to use antennas that have physical heights varying from 300 to800 feet when operating at 100 kHz. A technique for overcoming thelimitations of such electrically short radiators is the use oftop-loading which effectively increases the electrical height of theantenna while leaving its physical height unchanged.

Top-loading can take the form of a large plate, either square orcircular in shape, mounted on top of the antenna or it can take the formof an umbrella. Umbrella top-loading comprises the use of the three ormore guy wires as loading elements. It is not uncommon to utilize asmany as 12 guy wires as loading elements. These guy wires areelectrically connected to the top of the tower and their length is fixedby the insertion of guy insulators at desired points in the guy wires.Adjusting the length of the loading wires and the angle at which theyextend from the tower will control the radiation resistance and thereactance of the antenna which in turn directly affects the efliciencyand the dynamic bandwidth of the antenna systern. Although the dynamicbandwidth of the antenna system is materially increased from that of thesame antenna without top-loading, the bandwidth is not controllable,that is, a given bandwidth will be obtained for a given length and angleof top-loading guy wires in relation to the height of the tower. Theonly way in which the bandwidth can be increased therefore is tophysically increase the height of the tower and/or top-loading orinserting a loss resistor in series with the antenna feedpoint in orderto broadband the antenna. The latter expedient, however, results inreduced efficiency. One type of antenna system which discloses a meansto control the bandwidth and efficiency for a short length (height)vertical antenna is disclosed in U.S. Ser. No. 502,852, now Patent No.3,386,098, entitled Electrically Short Tower Antenna With ControlledBase Impedance, filed Oct. 23,

3,482,249 Patented Dec. 2, 1969 C&

1965, in the name of John H. Mullaney and assigned to the assignee ofthe present invention.

It is an object of the present invention therefore to provide animproved short length antenna system in which the radiation efiiciency,bandwidth and driving point impedance are controllable;

It is another object of the present invention to provide a series-fed,top-loaded antenna system suitable for wide band, low frequencycommunications;

It is yet another object of the present invention to provide an improvedmonopole antenna system utilizing multiple tuning techniques forincreasing the efiiciency and bandwidth of electrically short verticalantennas.

Briefly, the subject invention contemplates top-loading a series-fedmonopole antenna having a vertical radiating tower which is mounted on abase insulator with the toploading comprising a plurality of top-loadingconductors connected in a predetermined configuration to the top of theantenna and terminated at their distal extremities by means of variablecapacitors which are coupled to ground so that the top-loadingconductors may be stagger tuned. In addition, the present inventioncontemplates utilizing a fat monopole antenna having its base insulatedfrom ground comprising a vertical tower and one or more fold conductorslocated adjacent the tower, running substantially parallel thereto, andbeing coupled not only at the top of the tower but also at its base. Itis also desirable, in certain instances, that a second fold conductor belocated adjacent the antenna having one end connected to the top of theantenna with the other end terminated in a variable capacitor which isconnected to ground.

Other objects and advantages will become apparent after a study of thefollowing specification when read in connection with the drawingswherein like reference numerals represent like components, and

FIGURE 1a and FIGURE lb is a schematic diagram and equivalent circuit,respectively, of a series-fed, toploaded monopole antenna typical ofknown prior art antenna systems;

FIGURE 2 is a perspective view of a first embodiment of the subjectinvention;

FIGURES 3a and 3b disclose a schematic diagram and equivalent circuitdiagram, respectively, of the embodiment shown in FIGURE 1;

FIGURE 4 is a schematic diagram of a second or simplified embodiment ofthe subject invention;

FIGURE 5 is a schematic diagram of a third embodiment of the subjectinvention;

FIGURE 6 is a schematic diagram of a fourth embodiment of the subjectinvention;

FIGURE 7 is a schematic diagram of a fifth embodiment of the subjectinvention;

FIGURE 8 is a graphical illustration of the electrical characteristicsexhibited by selected embodiments of the subject invention; and

FIGURES 9 and 10 are illustrative of Smith chart representations of theelectrical characteristics of the selected embodiments.

Referring now more particularly to the drawings, FIGURE 1a discloses inschematic form what is considered to be known prior art apparatus. It iscomprised of a vertical radiator or tower 12 mounted on a base insulator14 which has one side or terminal returned to a point of referencepotential 16 hereinafter referred to as ground. A feed point 18 iscoupled to the opposite side of the base insulator 14 which is common tothe base of the tower 12. A radio transmitter source, not shown, isadapted to be coupled to the feed point 18. This configuration is knownto those skilled in the art as a seriesfed antenna. At the top of thetower 12 is a plurality of guy wires 20 extending outwardly therefrom ata predetermined angle and terminated in guy insulators 22. The

top-loading of the antenna system thus established is referred to asumbrella top-loading. Electrically, the antenna system shown in FIGURE1a can be represented as a capacitor 24 in series with a resistance 26as shown in FIGURE lb where R is the radiation resistance exhibited bythe antenna and the capacitor 24 is representative of the distributedcapacity of the tower and top-loading elements.

In such an antenna, the resistance value R is characteristically verylow while the capacitive reactance X is very large. The Q of an antenna(a figure of merit indicative of the sharpness of the resonance curve)is approximately equal to the base reactance (X) divided by theradiation resistance (R). An antenna of the type shown schematically inFIGURE la therefore has relatively narrow bandwidth and is extremelyselective. The effect of the top-loading is to increase the radiationresistance while decreasing the reactance, i.e., increasing thecapacitance of the structure. This lowers the Q which in turn increasesthe bandwidth.

It should be noted for purpose of explanation that the bandwidth isgenerally defined as the frequency difference between the upper andlower frequency points where 50% of the applied power is delivered tothe antenna. Another way of expressing it is by saying the bandwidth iscommonly considered to be the frequency band within which the power isequal to or greater than one-half the power radiated at resonance. Thebandwidth of an antenna depends upon its input impedance and the ratewith which its reactance and resistance changes with frequency. Thereare two types of bandwidths to be considered. One is the staticbandwidth which is the antenna reactance divided by two times theantenna radiation resistance and is the bandwidth which would beobtained if the antenna system had no losses. The other is the loaded ordynamic bandwidth which is the net bandwidth after consideration isgiven to total antenna system losses and the reactance used to resonatethe antenna. The loaded or dynamic bandwidth considerations can only beobtained by considering the coupling components used to resonate theantenna. For example, considering the antenna shown schematically inFIGURE 1a, to resonate this antenna, it would be necessary to cancel outthe capacitive reactance. This requires an inductive reactance equal inmagnitude to the capactive reactance coupled to the circuit.Furthermore, the bandwidth is not controllable, that is, a givenbandwidth will be obtained for a given length of top-loading, angle oftop-loading guy wire, and height of the radiator.

Considering the present invention in detail, attention is called toFIGURE 2 which discloses a first embodiment of the invention andcomprises a vertical radiator 12 in the form of a radiating towermounted on and insulated from ground by means of a base insulator 14. Aradio transmitter 28 is coupled across the base insulator 14 by means ofthe feed point 18 which is coupled to the common connection between baseof the tower 12 and the insulator 14. The transmitter 28 then acts tofeed the vertical radiator 12 at its base 15. A fold conductor 30,hereinafter referred to as a told, is located adjacent the verticalradiator 12 such that it is situated substantially parallel theretobeing held from the tower by means of stand-off insulators 32 a smalldistance, for example, from three to five feet. One end of the fold 30is attached or connected to the top of the radiator tower 12 at theterminal 34 while the opposite end of the fold is directly connected tothe base 15 of the tower 12 which is also common to the insulator 14 andfeed point 18. It would appear that such a connection would act to placeashort circuit across the vertical radiating tower 12 but, in effect, itacts to increase the effective diameter of the vertical radiator so asto provide what might be termed a fat monopole antenna. Threetop-loading conductors 36 are connected to the top of the tower by meansof the terminal 34 and are equally spaced in azimuth about the verticalaxis of the tower such that they extend outwardly at a predeterminedangle therefrom towards respective masts 38 and guy wire insulators 40.The top loading conductors 36 are not terminated at the insulators 40 asin prior art systems but extend downwardly substantially parallel to themasts 38 in the form of drop wires 36'. These drop wires 36' are alsospaced away from their respective masts 38 by means of stand-offinsulators 42. The distal ends of the drop wires 36' are terminated invariable capacitors 44.

What is accomplished by the present invention is to transform an antennasystem which has an inherently high Q such as shown in FIGURE 1a into anantenna system which has a low Q. Utilizing the configuration as shownin FIGURE 2 wherein a fat monopole antenna is achieved, the ratio of thelength to the diameter (L/D) of the antenna is decreased because ineffect the fold acts to effectively enlarge the diameter of the verticalradiator. Keeping in mind the relationship that Q=X/R, thecharacteristic of the fat monopole is one which has both a lowerradiating resistance and a lower reactance; however, the decrease inresistance is not as significant as the decrease in reactance;therefore, the Q of the circuit is decreased. The effect of thetop-loading elements effectively increases the electrical length orheight of the vertical radiator which effectively increases theradiation resistance while reducing the reactance X even more, thuseffectively lowering the Q of the circuit even further. As notedearlier, the effect of top-loading such as shown in FIGURE la is to addcapacity to the top of the structure. In the subject invention, thecapacitance as eX- hibited by the top-loading is further enhanced andmade controllable by the efiect of the combination of the drop wires 36and the variable capacitors 44, respectively. By means of the capacitors44 the top-loading can be controlled at will. By selectively tuning thetop-loading elements by means of capacitors 44, the radiationefficiency, the bandwidth, and the driving point impedance arecontrolled so as to optimize the desired operating characteristics ofthe antenna system. When desirable, the tuning can be obtained by meansof the dro wire 36 alone, i.e., varying the length and spacing from themast of each drop wire 36. It is contemplated that the preferable methodof tuning the top-loading is to stagger tune the top-loading elements inorder to provide a symmetrical bandwidth characteristic. Althoughstagger tuning is desirable, it is not absolutely necessary. Thesecharacteristics will be discussed in greater detail subequently withreference to FIGURES 8, 9 and 10.

FIGURE 3a is a schematic diagram of the antenna system of the embodimentshown in FIGURE 2. The height H of the vertical radiating structure ortower 12 is in the order of 520 (360 being a full wavelength). The fold30 is connected both to the base and the top of the vertical radiator 12and the antenna system is series-fed by means of the feed point 18coupled to the common connection between the base insulator 14 and thebase 15. The top-loading elements are schematically represented by threevariable capacitors 46 which is meant to include the combined capacityof each top-loading condoctor 36, the drop wire 36' and the variablecapacitor 44. Although three top-loading elements are shown, any numbermay be utilized when desirable. Increasing the number of top-loadingelements merely decreases reactance while increasing the radiationresistance.

The equivalent electrical circuit of the embodiment shown in FIGURE 2 isshown in FIGURE 3b wherein the base insulator 14 is represented by acapacitance 48 while the resistance, inductance and capacitance of thevertical radiator or tower 12 is represented by reference characters 50,52 and 54, respectively. The resistance and inductance of thetop-loading elements, which is almost negligible, is represented by theresistance 56 and inductance 58. Connected in series therewith is thecombined variable capacitance 46.

Basically, what the equivalent circuit shown in FIG- URE 3 illustratesis a network which can be selectively tuned to resonance by properadjustment of the value of the capacitance 46 which is essentiallydetermined by the value of the variable capacitors 44 shown in FIG- URE2. As it is well known to those skilled in the art, the point at whichmaximum power is transmitted to the antenna system is at the point ofresonance wherein the inductive reactance is cancelled by the capacitivereactance so that the entire circuit substantially acts as a lossyresistor which radiates electrical energy.

Referring now to FIGURE 4, there is illustrated a second embodiment ofthe subject invention which discloses the simplest configurationcontemplated and schematically shows a series-fed monopole antenna whichhas its base insulated from ground. It is comprised of a verticalradiator or tower 12 devoid of a fold such as shown in FIG- URE 2 beingfed froma transmitter, not shown, by means of feed point 18 which iscoupled to the common connection between the base of the radiatingantenna and the base insulator 14. The configuration is top-loadedbythree folds, such as shown in FIGURE 2, having their distal endsterminated in variable capacitors, such as capacitors 44, with thecombined capacities being illustrated schematically as the variablecapacitors 46.

FIGURE 5 illustrates schematically a third embodiment of the subjectinvention which is similar to the embodiment shown in FIGURES 2 and 311.It is similar in all respects but additionally includes a variableinductance 50 coupled across the base insulator 14 to ground. Thevariable inductance is coupled to the common connection between the baseof the vertical radiator 12 and the base insulator 14 which is common tothe feed point 18 and the fold 30. The purpose of the variableinductance 50 is to provide still further control of the bandwidth anddriving point impedance of the antenna system. The addition of theinductance 50 has an inherent limitation in that it introducesadditional losses in the system which will in a slight manner reduce theradiation efiiciency; however, it is often desirable to sacrificeefliciency for further bandwidth control.

The embodiments of the present invention shown schematically in FIGURES6 and 7 additionally include another fold 52 which is located adjacentthe tower radiator 12 in a manner similar to the fold 30 shown in FIGURE2. The additional fold 52, moreover, runs substantially parallel to thetower 12 and is connected to the radiator at the upper end thereof atterminal 34 but is terminated at its other or distal end in a variablecapacitor 54 returned to ground.

FIGURE 6 illustrates a series-fed monopole antenna with adjustabletop-loading elements and includes the additional fold S2 terminated incapacitor 54. With respect to FIGURE 7, there is disclosed a fatmonopole antenna system which is series-fed comprising the verticalradiator 12 and a fold 30 coupled across the tower in addition to thesecond fold 52 which is connected in series with the capacitor 54 fromthe top of the radiator to ground. The purpose of the additional fold 52and the adjustable capacitor 54 is to provide still further control. Byadjusting the three termination capacitances 46 by means of theadjustable capacitors 44 forming a part thereof and adjusting the foldcapacitor 54 a more precise bandwidth is obtained while at the same timeproviding an exact driving point resistance at the desired frequency ofoperation.

The antenna configurations disclosed by the subject invention haveapproximately an omnidirectional radiation pattern in the horizontalplane; however, they can be directionalized to have a minimum-to-maximumradiation pattern of approximately 6 db by introduction of staggerturning of the variable capacitors as mentioned above; thatis, byselectively adjusting the values of capacity in the folds, both thetop-loading folds and the second folds when considering the embodimentsshown in FIGURES 6 and 7.

While the subject invention has been described as a means of obtainingrelatively wide bandwidth which is controllable as well as controlleddriving point impedance and radiation efficiency, it is of interest tonote the comparison of the band-width characteristics of the subjectinvention with that of the known prior art series-fed monopole antennashaving top-loading as shown in FIG- URE 1a. In this regard, attention isdirected to FIG- URES 8, 9 and 10. First, FIGURE 8 is a plot of therelative forward power vs. frequency for selected configurations takenfrom test data obtained in making comparative measurements of electricalcharacteristics at an operating frequency of 50 kHz. The curve a isrepresentative of the characteristics of the series-fed monopole antennawith top-loading which is typical of the prior art. This curve indicatesthat such antenna system has relatively narrow bandwidth and highselectivity. Curve b is a characteristic curve of the antenna systemsembodied by the subject disclosure in FIGURES 3a, 4, and 5. Noting thatthe bandwidth is the frequency difference where the radiation powerfalls off to 0.5, it can be seen that the bandwidth for the subjectinvention is approximately 20 kHz. whereas the prior art apparatus has abandwidth of approximately 1 kHz. Curve 0 is illustrative of bandwidthcharacteristic which can be obtained utilizing the embodiments shown inFIGURES 6 and 7 whereby the bandwidth can be increased to 35 kHz.

FIGURES 8 and 9 are further illustrative of the dynamic bandwidthcharacteristics of the above mentioned configurations when plotted on aSmith chart which is a coordinate system of two orthogonal families ofcircles, corresponding to constant standing wave ratio and to constantelectrical length, respectively, when superposed upon a rectangularcoordinate system in which relative reactances are plotted as ordinatesagainst relative resistance as abscissas. It can also be shown that whenan antenna has a VSWR (voltage standing wave ratio) of 5.83:1, theavailable power delivered to the antenna is one-half (0.5). This beingthe case, when the impedance versus the frequency of an antenna isplotted on a Smith chart, the useful bandwidth is readily determined byconsidering the portion of the curve that falls within a 5.83:1 VSWRcircle.

The experimental data acquired in actual tests made has been plotted onthe Smith charts shown in FIG- URES 9 and 10 (FIGURES 1, 2, and 4 ofdisclosure) and more precisely indicate the bandwidth characteristicsshown in FIGURE 8.

Referring now to FIGURE 9, curve a illustrates the 5.83 :1 VSWR circle.Curve b depicts the impedance characteristic of the prior art systemshown in FIGURE 1a. It will be observed that at the frequency of 50 kHz.the impedance is purely resistive and therefore resonant. The curve issemicircular in shape and intersects the 5.83:1 VSWR circle in thevicinity of the frequency 49.41 and 50.27 kHz. This indicates that thesystem includes reactive components on each side of 50 kHz. and havingbut one point of resonance. Furthermore, the bandwidth is less than 2kHz. wide, considering the intersection of the 5.83:1 VSWR circle. Curvec on the other hand is characteristic of the impedance of the subjectinvention and embodiments shown in FIGURES 3a, 4 and 5. Again it isresistive at 50 kHz.; however, the curve doubles back on itself andintersects the VSWR circle at 43.32 and 64.17 kHz. It has two otherresonant points where the curve crosses the abscissa and exhibits abandwidth of more than 20 kHz. This provides an improvement unobtainablewith the prior art embodiment shown in FIGURE 1.

With respect to the Smith chart shown in FIGURE 10 curve d is indicativeof the embodiments shown in FIG- URES 6 and 7 which exhibit a stillwider bandwidth inasmuch as the curve folds back upon itself twiceintersecting the 5.83:1 VSWR circle (curve a) at approximately 44.96 and79.72 kHz. A bandwidth of approximately 35 kHz. is obtained.

The bandwidths obtainable by means of the present invention are farsuperior to those measured on a simple top-loaded, series-fed antenna ofthe same physical height. Inasmuch as the bandwidth also is a functionof the L/ C ratio (the inherent inductance of the antenna structure andits top-loading wires plus that introduced by the guy termination and/orfold capacity to ground), it necessarily follows that the subjectinvention is an appreciable improvement inasmuch as there are no tunableor adjustable reactance elements in the prior art top-loaded structurefor controlling bandwidth.

The present invention moreover provides the following advantages: Itpermits the heretofore very low resistance of the antenna to be adjustedto a value of 50 ohms or higher for ease of coupling; it increases thedynamic bandwidth for a given L/ C ratio to a controlled bandwidthsuitable for wide band communications; it permits the drive pointcurrent to be lower for higher power inasmuch as the drive pointresistance has been raised to a value, for example, 50 ohms, whereheretofore it has been in the order of 2 ohms or less.

What has been described in the present invention is an improvement inseries-fed, monopole antennas for obtaining a reasonable drive pointresistance and bandwidth for electrically short vertical antennas. Inaddition, the present invention provides a means for using high powerwith low feed point current and voltage.

While there has been shown and described what is at present consideredto be the preferred embodiments of the invention, other modificationswill readily occur to those skilled in the art. It is not desiredtherefore that the invention be limited to the specific arrangementsshown and described, but it is to be understood that all equivalents,alterations and modifications within the spirit and scope of the presentinvention are herein meant to be included.

I claim as my invention:

1. An antenna system in which the radiation efiiciency, bandwidth, anddriving point impedance are controllable, comprising in combination;antenna base insulator means; a series-fed, top-loaded moopole antennamounted on said antenna base insulator means and having a verticalradiator which is of a relatively short length (height) in comparison tothe Wavelength of operation; a feed point coupled to the commonconnection of said base insulator means and said vertical radaitor; aplurality of top-loading conductors commonly coupled to the upper end ofsaid vertical radiator and extending outwardly and towards the groundplane; a series reactance connected from the distal end of each of saidplurality of top-loading conductors to ground for tuning said antennasystem so as to obtain relatively wide bandwidth, high radiationefficiency and matched driving point impedance; and a fold mountedadjacent said vertical radiator in a substantially parallelconfiguration incluing means for coupling said fold to said upper endand said common connection of said base insulator means and saidvertical radiator.

2. An antenna system as set forth in claim 1 and additionally includingan inductance coupled across said base insulator means so as to have oneterminal coupled to said feed point and the opposite terminal coupled toground.

3. The antenna system as defined by claim 1 wherein the length of thevertical radiator is of the order of 5 20 and said series reactancecomprises a capacitance of a selected value whereby said top-loadingconductors are staggered tuned.

4. The invention as define-d by claim 1 and additionally including aseries capacitive reactance connected from the distal end of each ofsaid plurality of top-loading conductors to ground.

5. The invention as defined by claim 4 wherein said series capacitivereactance is of a selected value for staggered tuning said top-loadingconductors.

6. The antenna system as claimed by claim 4 and additionally including asecond fold mounted adjacent said vertical radiator in a substantiallyparallel configuration therewith and including means for coupling oneend of said fold to said upper end of said vertical radiator and aseries capacitance coupled to the opposite end of said second fold, withmeans terminating said capacitance to ground.

7. An antenna system in which the radiation efiiciency, bandwidth, anddriving point impedance are controllable, comprising in combination;antenna base insulator means; a series-fed, top-loaded monopole antennamounted on said antenna base insulator means and having a verticalradiator which is of a relatively short length (height) in comparison tothe wavelength of operation; a feed point coupled to the commonconnection of said base insulator means and said vertical radiator; aplurality of top-loading conductors commonly coupled to the upper end ofsaid vertical radiator and extending outwardly and towards the groundplane; a series reactance connected from the distal end of each of saidplurality of top-loading conductors to ground for tuning said antennasystem so as to obtain relatively wide bandwidth, high radiationefficiency and matched driving point impedance; and fold mountedadjacent said vertical radiator and running substantially parallelthereto and including means for coupling one end of said fold to saidupper end of said vertical radiator and means for coupling the distalend of said fold to a capacitive reactance which has one terminalconnected to ground.

8. An antenna system as set forth in claim 7 in which the length of saidvertical radaitor is of the order of 5 20 and said series reactancecomprises a capacitance of a selected value whereby said top-loadingconductors are staggered tuned.

9. An antenna system in which the radiation efficiency, bandwidth, anddriving point impedance are controllable, comprising in combination;antenna base insulator means; a series-fed, top-loaded monopole antennamounted on said antenna base insulator means and having a verticalradiator which is of a relatively short length (height) in comparison tothe wavelength of operation; a feed point coupled to the commonconnection of said base insulator means and said vertical radiator; aplurality of top-loading conductors commonly coupled to the upper end ofsaid vertical radiator and extending outwardly and towards the groundplane; a series reactance connected from the distal end of each of saidplurality of top-loading conductors to ground for tuning said antennasystem so as to obtain relatively wide bandwidth, high radiationefiiciency and matched driving point impedance; 2. fold conductorlocated adjacent and substantially parallel to said vertical radiatorand having one end thereof connected to the upper end of said verticalradiator, an adjustable capacitive reactance connected to the oppositeend of said fold conductor and being terminated at ground; and whereinsaid series reactance connected from the distal end of each of saidplurality of top-loading conductors comprises an adjustable capacitivereactance for selectively staggered tuning said antenna system.

References Cited UNITED STATES PATENTS 2,048,726 7/1936 Bohm 343-854 X2,283,618 5/1942 Wilmotte 343-850 X 2,998,604 8/ 1961 Seeley 343-874 XELI LIEBERMAN, Primary Examiner US. Cl. X.R.

